The present invention relates to control systems for variable displacement hydraulic units, and more particularly to such control systems which operate in response to electrical input signals.
Hydrostatic transmissions, consisting of a variable displacement pump and either a fixed or variable displacement motor, would be a typical use for the control system of the present invention. The variable pump typically has a pair of stroking cylinders and a charge pump which generates control pressure. The control system communicates control pressure to either the first or second stroking cylinder to displace the swash plate of the pump, from neutral, in either a first direction or a second direction. Typically, the first direction of operation of the pump would correspond to forward movement of the vehicle, whereas the second direction of operation would correspond to reverse movement of the vehicle.
The most common type of electrohydraulic servo for controlling the flow of control pressure to the stroking cylinders has been the nozzle-flapper servo, which provides precise, responsive control, but is quite expensive.
It is also known to those skilled in the art to use various arrangements of solenoid valves, both the ON-OFF type, and the proportional type. See for example U.S. Pat. No. 3,529,422. Such systems typically suffer from one of two possible disadvantages. Either the system utilizes two proportional solenoid valves, in which case the system again becomes quite expensive, or only the ON-OFF type solenoid valves are used, in which case the system is typically arranged such that one of the solenoid valves becoming stuck in the open position may result in the vehicle operator being unable to bring the pump back to neutral, and bring the vehicle to a safe stop.